1. Field of the Invention
The present invention relates to electrodes for lithium ion secondary (i.e., rechargeable) cells. In particular, the present invention relates to the fullerene-based anodes for lithium ion secondary batteries.
2. Description of the Related Art
Lithium ion secondary cells or batteries are commonly used as power sources in portable electronic devices. Such rechargeable cells generally use a lithium transition metal oxide (e.g., lithium colbaltate) positive electrode and a negative electrode composed of a highly porous carbonaceous material, typically graphite. However, the carbonaceous material may also include other carbons, metal and/or a pyrolyzed organic material. A lithium ion-soluble electrolyte is provided between the two electrodes, and the cell is charged. During the electrochemical process of charging, some of the lithium ions in the positive electrode migrate from the positive electrode (serving as the anode) and intercalate into the negative electrode (serving as the cathode). During discharge, the negative charge held by the negative electrode (now serving as the anode) is conducted out of the battery through its negative terminal and the lithium ions migrate through the electrolyte and back to the positive electrode (now serving as the cathode). While it is understood that the terms “anode” and “cathode” apply to each of the negative and positive electrodes depending upon whether the cell is being charged or is discharging, hereinafter the term “anode” is used to refer to the negative electrode and the term “cathode” is used to refer to the positive electrode.
Lithium ion batteries possess a number of advantages over other rechargeable cells, such as nickel metal hydride (Ni-MH), nickel-cadmium (Ni—Cd), lead-acid, and lithium cells. For example, the use of lithium in lithium ion cells provides a higher specific energy density for these cells, about three times more than Ni—Cd and lead-acid batteries and about two times more than Ni-MH batteries. The use of lithium also provides a higher cell voltage of up to 4 volts. In comparison, Ni—Cd and Ni-MH batteries provide about 1.2 volts, and lead-acid batteries provide about 2 volts. Lithium ion batteries have a much longer shelf life, self-discharging at about ¼ to about ½ the rate of Ni—Cd, Ni-MH and lead-acid batteries, and also experience little or no memory effect. Further, lithium ion batteries are relatively environmentally benign, since they contain no lead, cadmium or mercury, and are considered safer than rechargeable lithium cells due to their use of lithiated carbon material in place of metallic lithium.
While the current lithium ion battery technology is a considerable improvement over other secondary battery technologies (e.g., batteries employing nickel and lead), there are some disadvantages associated with such batteries. In particular, while lithium ion batteries with graphite anodes exhibit a high initial reversible capacity, the reversible capacity of the batteries rapidly fades over a number of charging and discharging cycles. While not being bound by any particular theory, this rapid capacity fade is believed to be caused by exfoliation of graphite and/or irreversible electrochemical reduction of electrolyte that occurs to form a solid electrolyte interphase (SEI) layer at the anode. Thus, the use of conventional lithium ion batteries is not suitable for use in applications requiring long term battery usage involving a large number (e.g., several thousands) of charging and discharging cycles.
It is therefore desirable to improve the cycling characteristics of lithium ion batteries so as to maintain high reversible capacity values (e.g., at or close to the theoretical value for graphite of 372 mAh/g) as well as high intercalation/deintercalation efficiencies of lithium ions over multiple battery cycles.